I think there are 3 clear main issues with the 400Z.
1. The color they revealed it in was just a down right bad choice. It hid what lines and curves it has and fails to pop in pictures. If you look at the videos of it driving in the sunlight you can see the lines better and it looks a little cleaner, but it definitely would look better in almost any other color.
2. The front bumper/grille is a joke of design when looking head on. The rectangle look doesn't work and will never work with a modern design. I think the overall size of it is okay. The issue is, it's literally just a rectangle. Take a look at other sports/super cars with huge grille like the new Q60 or new Audi R8. They have a massive grille but it works and looks aggressive because it's more of a hexagonal shape.
3. Trying to modernize the older retro look of past Zs just doesn't entirely work here, IMO. I think they tried too hard to literally copy and past some of the designs when they should've just redesigned the look and then incorporated subtle retro styling cues. I do like the headlights. I feel like the tail lights could've been executed better. When I look at that long black strip of paint between the taillights I almost imagine the Toyota Prius rear. Maybe changing that black rectangle to body color would look better. Also, giving the tail lights a more 3D, unique shape instead of the flat, blocky appearance it has now, would really help.
Overall, I think they could've been a bit more creative with certain elements without breaking the bank. The interior is perfectly fine for me. The side profile looks great. Just tweak the front grille/bumper and the rear a bit and I'll take one hands down.
This video explains exactly my thought process on why the front grille needs a redesign.